The present invention relates generally to registration control systems used for printing presses and in particular to registration control systems operating on photoelectric principles.
It is important in virtually all printing applications to maintain registration between various print stations which might apply different colors, sizes or types of print as well as between print stations and other controls such as folding devices or page cutoff shears. Minute errors in registration can cause blurred images, ghosts, page displacement and other unacceptable printing conditions. While the various press work stations, such as printers, dryers, folding devices and cutting shears generally operate in close synchronism with each other by virtue of their common drive system, the longitudinal position of the web itself may vary with slippage, slackening and various environmental factors, such as humidity and temperature, that may cause shrinkage or stretching of the web.
Therefore, numerous systems have been devised for controlling the longitudinal registration between a moving web and the various work stations of a printing press. Several of these known registration systems employ scanning devices for comparing repetitively occurring marks printed on the web to the position of various work stations in their operating cycle. The work station drive mechanism is often used as a reference for this comparison, and to this end, various devices have been used to generate a "window" or monitoring period in each cycle of the work station during which the web is optically scanned for registration marks and position errors generated. A common method for generating this window is through the use of various press driven discs having apertures which are photoelectrically or magnetically detectable to define the window area. In systems of this type, however, the observation window is generally fixed relative to the cyclical position of the work station, such as a web shearing drum, and the web markings must be carefully positioned to coincide with the observation window during initial setup of the press. Still other systems employ shaft encoders and devices to index the encoder to the cycles of the work station. These systems also have entailed a difficult setup procedure and require a careful initial positioning of the web scanning devices either manually or through expensive and complicated automatic controls.
More important, the systems heretofore known in the art are generally somewhat inflexible in their ability to synchronize or lock onto a given mark in the print pattern on the web and they often require that the mark be distinctive and totally separated from other portions of the print pattern. As such, they present varying degrees of difficulty during initial setup that have to be accommodated. Many of these systems are analog in nature and, as such, are inherently limited in their ability to distinguish between marks of varying intensity on a given print pattern. Their use is limited accordingly.